1. Field of the Invention
This invention relates to an apparatus for deep frying food products.
2. Related Background Art
Liquid heating apparatus generally rely on either an electric element disposed in the liquid to be heated or a low efficiency heat exchanger which indirectly heats the liquid by means of gas or electricity.
Such known apparatus are not energy efficient due to the many thermal interfaces involved in the process, they are expensive to run and in general occupy a relatively large amount of space.
It is therefore an object of the present invention to provide a liquid heating apparatus which is inexpensive to run and which does not occupy a large amount of space.
Another disadvantage of known liquid heating apparatus is that there is often an uneven temperature distribution throughout the heated liquid and this problem is particularly apparent in large heating vessels. Pumps are known which can be used to pump the heated liquid to evenly distribute the temperature. Another advantage of providing a pump is that the heated liquid can be distributed or passed though a treatment element such as a filter. However, the inclusion of a pump in the apparatus adds to the cost and physical size of the apparatus.
Many liquids such as wax and cooking fat solidify or become extremely viscous when cool and a problem with this is that the rotation of the impeller of any pump in the liquid will be inhibited when the liquid is cold. This can damage the motor which drives the impeller.
It is therefore an object of the present invention to provide a liquid heating apparatus which is able to provide an even temperature distribution throughout the liquid and which avoids the above problems associated with conventional circulation pumps.
In accordance with this invention, there is provided a liquid heating apparatus comprising an electrically inductive impeller disposed in a chamber arranged to contain the liquid to be heated, drive means arranged to rotate the impeller to induce a flow in the liquid in the chamber, and an electrical coil disposed adjacent the impeller and arranged to induce eddy currents therein.
In use, a high frequency signal (in excess of 20 kHz) is applied to the coil, which generates a magnetic field that induces eddy currents in impeller. The impeller is not an ideal conductor, and thus the electrical energy is dissipated as heat as current flows through the impeller. Thus, the heating effect is proportional to I2R, where I is the current in the impeller and R is the electrical resistance of the impeller.
The resistivity of the impeller depends on the material that it is made from. Thus, it will be appreciated that the temperature which the impeller reaches will be dependent on the material of the impeller. The impeller directly heats the liquid and thus the apparatus is efficient. The impeller also acts to circulate the liquid and thus an even temperature distribution can be achieved without the requirement for a pump and separate heating element. The impeller can also be used to distribute the heated liquid or to pass it through a treatment element such as a filter. The apparatus will not be damaged if the material to be heated is of the kind whose viscosity is inversely proportional to temperature by virtue of the fact that the impeller rapidly heats up, thereby quickly heating the surrounding liquid and allowing the impeller to rotate normally. The impeller helps to distribute the locally heated liquid around the apparatus so that all of the material soon becomes fully flowable.
In a preferred embodiment, means may be provided for energizing the coil prior to rotation of the impeller, so as to reduce any risk of damage to the drive means before the surrounding material becomes fully flowable.
Many liquids expand as they change in temperature and it will be appreciated that this can damage the apparatus. Accordingly, preferably a wall of the chamber is resiliently deformable in order to allow expansion of the liquid as it changes in temperature.
Preferably the coil is disposed outside the chamber on an opposite side wall thereof to the impeller.
Preferably the wall is formed of a magnetically permeable material such as plastics or glass.
The amount of power required to heat a liquid is much greater than that for a gas and thus a large current has to be applied to the coil in order to quickly heat the liquid. furthermore, the temperature to which the liquid is to be heated is often high and this again necessitates a large coil current.
A disadvantage of large coil currents is that the coil itself can become very hot and potentially damaged due to I2R losses. This problem is exacerbated by the heat radiating from the heated liquid within the chamber. In order to overcome this problem, the coil is preferably separated from the wall of the chamber by an insulating layer of magnetically permeable material.
Preferably the layer of magnetically permeable material comprises air. Preferably a fan is provided for causing flow of the air in said layer.
Preferably the windings of the coil are open. Preferably the fan causes a flow of air through the coil windings.
Preferably the impeller is driven by a shaft, the fan being mounted on said shaft.
At high frequencies in the order of those used in the present invention, the current is confined to the skin of the coil winding owing to the so-called skin effect. This has the result of reducing the effective cross-sectional area of the winding carrying the current. Hence, the heating of the coil is further increased due to the corresponding increase in resistance of the coil. In order to overcome this problem, the coil preferably comprises windings which each comprise a plurality of electrically insulated conductors connected in parallel.